Well, about a century ago, when I got married, we weren't thinking about ethical dilemmas, because we weren't really aware of them back then. I gave my wife a diamond that had been my grandmother's, partly because the stone was free to me (don't tell my wife that!), and partly because the family history made it a bit more special than just some random diamond we found in a store.
I suspect, as family sizes stabilize on two or fewer kids, there's going to be a lot more of passing down family diamonds, and therefore less demand for new...
Second-hand synthetic diamonds, sure - but buying anything second hand is generally more morally defensible than buying that thing new.
Buying a used natural diamond is still worse than buying a synthetic one, because you're propping up the resale value of natural diamonds and therefore encouraging other people to buy them.
